1. Field of the Invention
The present invention relates to a high-frequency switch, and particularly, relates to a high-frequency switch for switching a signal path in a high-frequency circuit, for example, a digital portable telephone or the like.
2. Description of the Prior Art
As shown in FIG. 5, a high-frequency switch is used for switching a connection between a transmitting circuit TX and an antenna ANT and a connection between a receiving circuit RX and the antenna ANT in a digital portable telephone or the like.
FIG. 6 is a circuit diagram showing one example of a high-frequency switch which is a background of the present invention and to which the present invention is applied. The high-frequency switch is connected to an antenna ANT, a transmitting circuit TX and a receiving circuit RX. To the transmitting circuit TX, an anode of a first diode D1 is connected via a first capacitor C1. The anode of the first diode D1 is grounded via a series circuit of a first strip line SL1 and a second capacitor C2. Furthermore, to an intermediate point of the first strip line SL1 and the second capacitor C2 , a first control terminal T1 is connected via a first resistor R1. To the first control terminal T1, a control circuit for switching the high-frequency switch is connected. Also, a cathode of the first diode D1 is connected to the antenna ANT via a third capacitor C3 . To the third capacitor C3 connected to the antenna ANT, the receiving circuit RX is connected via a series circuit of a second strip line SL2 and a fourth capacitor C4. Also, to an intermediate point of the second strip line SL2 and the fourth capacitor C4, an anode of a second diode D2 is connected. Then, a cathode of the second diode D2 is grounded.
When transmitting with the high-frequency switch shown in FIG. 6 , a positive voltage is applied to the first control terminal T1. By this voltage, the first diode D1 and the second diode D2 are turned ON. At this time, a D.C. component is blocked by the first-fourth capacitors C1-C4, so that the voltage applied to the first control terminal T1 is applied to only a circuit including the first diode D1 and the second diode D2. When the first diode D1 and the second diode D2 are ON, a signal from the transmitting circuit TX is transmitted to the antenna ANT, and then the signal is transmitted from the antenna ANT. Meanwhile, the transmitting signal from the transmitting circuit TX is not transmitted to the receiving circuit RX, since the second strip line SL2 is grounded by the second diode D2 and resonates, and an impedance observed from a connecting point A to the receiving circuit RX side is a very large.
On the other hand, when receiving, no voltage is applied to the first control terminal T1, so that the first diode D1 and the second diode D2 are OFF. Thus, a receiving signal from the antenna is transmitted to the receiving circuit RX and is not transmitted to the transmitting circuit TX side. In this way, by controlling a voltage applied to the first control terminal T1, transmitting and receiving can be switched.
FIG. 7 is a plan view showing one example of a conventional high-frequency switch having the circuit shown in FIG. 6. The high-frequency switch 1 includes a substrate 2, first and second strip lines 3a and 3b and lands are formed on one main surface of the substrate 2. First and second diodes 4a and 4b, first, second, third and fourth chip capacitors 5a, 5b, 5c and 5d, and a first chip resistor 6 are connected to the strip lines and the lands.
However, in the conventional high-frequency switch 1 shown in FIG. 7, it is generally required for the first and the second strip lines 3a and 3b, to have a length of 1/4 of a wavelength of a transmitting signal or a receiving signal. A length of several tens of mm is required though it depends on a dielectric constant of the substrate 2 and so on. Thus a portion associated with the first and the second strip lines 3a and 3b has a large area on the substrate 2. Thus, in the high-frequency switch 1, it is a problem to miniaturize.